1. Technical Field
The present invention relates to signal transmission methods for transmitting pulse-width modulation signals, drive circuits, electro-optical devices, and electronic apparatuses.
2. Related Art
In printers used as image forming apparatuses, a light-emitting device including many light-emitting elements arranged in an array is used as a head device for forming electrostatic latent images on an image carrier, such as a photosensitive drum. Such a head device is usually formed of a plurality of light-emitting elements arranged in a line in a main-scanning direction. In addition, light-emitting diodes, such as organic light-emitting diodes (hereinafter, may be referred to as “OLEDs”), are widely used as light-emitting elements.
A head device includes a plurality of pixel circuits and a drive circuit that are provided on a substrate. Each of the pixel circuits includes an OLED element and a driving transistor provided near the OLED element for supplying a drive current to the OLED element. As a method for driving OLED elements, a PWM (pulse-width modulation) driving method is known (for example, see JP-A-2002-108285). In the PWM driving method, a constant current is supplied to an OLED element when a driving transistor is in an ON-state, and a grayscale level is determined in accordance with setting of an ON-state or an OFF-state.
In the PWM driving method, the grayscale level of an OLED element is determined in accordance with the pulse width of a pulse-width modulation signal. Thus, in order to increase the number of grayscale levels, it is necessary to reduce the width of a pulse. However, a reduction in the pulse width increases the operation frequency of a drive circuit.
In addition, since many OLED elements are provided in an optical head, wiring lines for transmitting many pulse-width modulation signals to driving transistors are required.